In this Sept. 24, 2013 file photo, Turkish Islamic preacher Fethullah Gulen is pictured at his residence in Saylorsburg, Pa. A lawyer for the Turkish government, Robert Amsterdam, said that (AP Photo/Selahattin Sevi)

A Turkish prosecutor on Friday charged 28 suspects including US-based Muslim preacher Fethullah Gulen, blamed by Ankara for a 2016 failed coup, over the murder of the Russian ambassador later in the same year.

The indictment by the Ankara public prosecutor came nearly two years after Andrei Karlov was shot dead in December 2016, state-run news agency Anadolu reported.

Turkey accuses Gulen of ordering the July 15, 2016 attempted putsch, a claim he strongly denies.

Karlov was shot dead by an off-duty Turkish policeman Mevlut Mert Altintas, 22, while inaugurating a photo exhibition in Ankara.

Altintas was killed shortly after the murder by members of Turkey's special forces.

The 28 suspects are charged with "violating the constitutional order", "being a member of a terror organisation", "premeditated murder with the intention of causing terror", and "seeking to create terror or panic", Anadolu reported.

The prosecutor sought varying terms including aggravated life sentences, which have replaced the death penalty in Turkey and carry harsher conditions than normal life imprisonment convictions.

Turkish authorities have repeatedly pointed the finger at Gulen and his "Hizmet" (Service) in Turkish over the killing, and said the murder was plotted by the group.

In April this year, a Turkish court issued arrest warrants for eight people including Gulen over the murder.

Among those sought by Ankara and also named in Friday's indictment was Serif Ali Tekalan, who headed a university linked to Gulen in Istanbul and now heads the Texas-based North American University (NAU).

Ankara refers to the Gulen movement as the "Fethullah Terrorist Organisation" (FETO) and in the indictment, the prosecutor said the group was a spy and intelligence organisation that wanted to cause a "provocation" between Turkey and Russia with the murder.

Moscow and Ankara had improved ties in mid-2016 after relations were damaged following the shooting down of a Russian plane by a Turkish military jet in November 2015 over the Turkey-Syria border.

Although Turkey has issued multiple arrest warrants for Gulen, the United States' failure to extradite the Pennsylvania resident has been a source of tension between NATO allies Ankara and Washington.